The use of medical imaging involving radiation is an integral part of medicine. But how safe are the effective radiation doses used in heart tests? By Michael Lim
THE recent nuclear power plant accident in Fukushima has raised public awareness and concern about radiation. Radiation is present everywhere. You can be exposed to radiation whether you are taking a walk out in the sun, flying overseas or undergoing medical testing. However, not all radiation is the same. What we are concerned about is “ionising” radiation which has the potential to harm our bodies. To help standardise the measurement of the net biological effect of radiation, the unit of measurement Sievert (Sv) is used internationally to quantify the effective dose (ED) of radiation. A millisievert (mSv) is a thousandth of a Sievert.
Mr A was an overseas patient who had a borderline treadmill test result and was sent by his doctor to the hospital for a nuclear scan of his heart to look for evidence of insufficient flow to his heart. As the nuclear scan appeared mildly abnormal, he had a CT scan of his heart arteries which showed a significant narrowing of a major heart artery. He then had an invasive an-giogram of his heart arteries which confirmed the narrowing, and underwent stenting of the heart artery. He subsequently saw me for further review and in the ensuing discussion asked me about the radiation dose of the tests he had undergone.
The use of medical imaging involving radiation is an integral part of medicine. Among the common tests used for the assessment of heart disease, nuclear perfusion scan of the heart, computed tomography scan of the heart arteries ( CT coronary angiogram) and invasive coronary angiogram involve the use of radiation.
A typical nuclear perfusion scan of the heart muscle involves the administration of radioactive particles into the body by intravenous injection, after which the heart is scanned. The effective radiation doses can vary depending on the different radioactive particles used; from 3.3 mSv for a rubidium scan to 8 mSv for a technetium scan and to 18 mSv for a thallium scan. As this involves the injection of radioactive particles into the body, internal organs are also exposed to radiation.
CT coronary angiography involves the injection of iodine containing contrast agents into the arm vein followed by scanning of the heart arteries using a multi-slice CT (MSCT). Radiation doses vary considerably depending on the MSCT used and the centres doing the scan. A study of 50 hospitals using 64-slice MSCT showed an average ED of 12 mSv, with a sixfold difference between different centres (5 mSv to 30 mSv). On average, the ED using a standard protocol for 256 and 320 MSCT is about three quarters that of a 64- slice MSCT. Using radiation reduction protocols, the radiation dose can be reduced to 3 mSv or less.
Invasive coronary angiography is a procedure that involves the insertion of a tube into the artery of the leg or hand into the heart artery where X-ray images are taken after injection of iodine containing contrast. The ED can vary as much as a tenfold difference depending on the experience and the technique of the operator. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) cites a typical value of about 7 mSv. In complex procedures involving the opening of blocked arteries with balloon and stents and procedures to treat abnormal heart rhythms, the long duration for X-ray visualisation during the procedure may increase the radiation doses by 2 to 5 times this level.
Radiation and cancer risks
How do we put all this information into perspective? In the US, the average person receives about 3.6 mSv a year from background radiation. For 10,000 persons receiving 10 mSv of radiation from a medical test, an additional eight people will be at risk of cancer death. Looking at this from another perspective, comparable common day activities with a relative risk of one in a million chances of dying in the US include receiving 0.1 mSv of radiation (chest X-ray), smoking 1.4 cigarettes, eating 40 tablespoons of peanut butter, driving 40 miles in a car, and canoeing for six minutes. Risk comparisons show that radiation is a relatively small risk, when compared to risks we take every day.
Estimates of cancer risk as a result of radiation exposure are based on plant and animal studies, and there is a lot of uncertainty with these estimates. Studies on atomic bomb survivors show a relationship between radiation exposure and cancer incidence, including leukemia and solid tumours. However, in nuclear accidents where the radiation exposure is lower, the data is different. According to a 2006 WHO report, the Chernobyl accident caused 47 deaths from 1986 to the year 2004 among firemen and severely exposed persons. In addition, more than 4,000 developed thyroid cancer (1992 to 2002), of which less than one per cent died.
The conclusions of the IAEA 2005 Chernobyl Forum study are in line with the UNSCEAR 2000 report, which said that “apart from this (thyroid cancer) increase, there is no evidence of major public health impact attributable to radiation exposure 14 years after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality or in non-malignant disorders that could be related to radiation exposure”. As yet, there is little evidence of any increase in leukaemia, even among clean-up workers where it might be most expected.
Based on the data available, the estimated cumulative radiation that Mr A was exposed to was 32 mSv ( 8 mSv for technetium scan + 3 mSv for optimal protocol MSCT scan + 7 mSv for invasive coronary angiography and 14 mSv for a single vessel stenting). This is below 100 mSv, the ED of radiation where there is a definite risk of cancer.
Finally, for those who are still uneasy about radiation, there is a silver lining in the cloud. A study published last month comparing imaging of the heart arteries using the newer generation magnetic resonance imaging (MRI) scan with the MSCT scan showed comparable accuracy. Obviously, the major advantage of the MRI scan of the heart arteries is that there is no X-ray radiation and there is no increase in cancer risk. As always, if you are uncertain about what tests are appropriate, always check with your doctor.
You can be exposed to radiation whether you are taking a walk out in the sun, flying overseas or undergoing medical testing. However, not all radiation is the same.